The modulo operator will be your key here:

$in = 391;

$hour = sprintf("%02d", int($in / 3600));
$min = sprintf("%02d", int($in / 60));
$sec = sprintf("%02d", $in % 60);

print "$in .. $hour:$min:$sec\n";
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However, I agree with the idea of letting SQL handle the complexities of dates and times - being as it's in use anyway.

Moggs